Need to lose a few pounds? End a “Bad Romance” with the most famous pop star in the world.

That’s how Lüc Carl, bartender, SiriusXM DJ and former boyfriend of Lady Gaga, did it, according to his memoir, “The Drunk Diet: How I Lost 40 Pounds . . . Wasted.”

Carl and Gaga dated on and off between 2005 and 2010, and he was apparently left in the dust when the pop star’s career took off. She’s now linked to “Vampire Diaries” star Taylor Kinney.

The failed heavy-metal musician, now 31, says he was “living somewhere between drunk and hung over at all times” while dating the future star, then known simply as Stefani Germanotta.

“Fat and bloated,” he popped cheeseburgers like gum, smoked two packs of Camels a day and ended every night in a drunken stupor.

After a night of mixing Irish Car Bombs, bourbon, Scotch and beer, he phoned his girlfriend — Gaga goes unnamed in the book — while he “threw up for 10 minutes.” It was the last straw for the singer, who “packed her s–t and moved out West to pursue her dreams.”

When Gaga’s album “The Fame” took off — with songs like “LoveGame” and “Poker Face,” reportedly written about Carl — he became motivated to work out.

“I was fat and miserable, but her career was blossoming. I couldn’t even watch reruns of ‘Seinfeld’ without seeing her singing in her underwear,” he writes in his book, due out Tuesday.

“I was happy for her, of course, but I didn’t need my ex shoved down my throat. I basically had no choice but to run. And run I did.”

Carl ran his first marathon in Las Vegas in December 2010.

Even though the two are no longer an item, Gaga announced last year that she’s a devotee of Carl’s fitness regimen. Carl hasn’t been as generous, with veiled references to his now-25-year-old ex.

“I used to get more women when I was broke, flabby and completely wasted all the time — but those women were sloppy 22-year-olds, the kind of women who messed up my sheets and asked for cab fare in the morning,” he writes.